FIG. 16 is a plan view of a conventional acceleration sensor, which is disclosed in Japan Laid Open Hei5-133976, for example. FIG. 17 is a sectional view taken at a line XVII—XVII in FIG. 16.
In FIGS. 16 and 17, a reference numeral 101 indicates a substrate. A first detecting electrode 102, a second detecting electrode 103 and a driving electrode 104 are provided on the substrate 101.
A reference numeral 105 indicates a movable electrode. The movable electrode 105 is provided within a frame of a semiconductor material 106 by facing against the first detecting electrode 102, the second detecting electrode 103, and the driving electrode 104. The movable electrode 105 is elastically supported by a deformation 107. The movable electrode 105 has a weight 108 at one end (an end on the second detecting electrode 103 side herein).
A metal contact 109 extends to a doped region 110 through an oxide film, 111. The doped region 110 extends downward and is in contact with the first detecting electrode 102, the second detecting electrode 103 and the driving electrode 104. The first detecting electrode 102, the second detecting electrode 103 and the driving electrode 104 may be provided on a different glass substrate. Alternatively, the first detecting electrode 102, the second detecting electrode 103 and the driving electrode 104 may be formed within the semiconductor material 106 by using a junction isolation technology or an oxide film isolation technology. The first detecting electrode 102, second detecting electrode 103 and driving electrode 104 in FIG. 17 are pn junction isolation embedded electrodes.
Next, the principle of acceleration detection by using such a conventional acceleration sensor will be described. FIG. 18 is a diagram for explaining a measuring principle by using the conventional acceleration sensor.
All of the first detecting electrode 102, the second detecting electrode 103 and the movable electrode 105 are conductive and are located by facing against each other. Capacitances C1 and C2 are provided between the first detecting electrode 102 and the movable electrode 105 and between the second detecting electrode 103 and the movable electrode 105, respectively. One end of the movable electrode 105, which is elastically supported by the deformation 107, has the weight 108. Therefore, the movable electrode 105 is sensitive to an acceleration in a depth direction of the semiconductor material 106. Then, the movable electrode 105 is easy to twist with respect to an axis linking the deformation 107. In other words, when an acceleration is applied in the depth direction of the semiconductor material 116 as indicated by an arrow 112, the movable electrode 105 twists with respect to the axis linking the deformation 107. Due to the twist of the movable electrode 105, an interelectrode distance on the capacitance C1 side is larger while an interelectrode distance on the capacitance C2 side is smaller between the capacitances C1 and C2. Therefore, a capacity value of the capacitance C1 decreases while a capacity value of the capacitance C2 increases. By differentially detecting the capacitance changes, the applied acceleration can be measured. When an acceleration is applied in a direction opposite to the arrow 112, the movable electrode 105 twists in a direction opposite to the above direction. Then, the capacity value of the capacitance C1 increases while the capacity value of the capacitance C2 decreases.
The conventional acceleration sensor uses an inertial force acting on the weight 108 when an acceleration is applied thereto to convert the acceleration to a twist of the movable electrode 105 and to changes in capacitances C1 and C2 between the first and second detecting electrodes 102 and 103 and the movable electrode 105. Thus, the acceleration can be measured. Therefore, as shown in FIG. 18, an amount of change dl in interelectrode distance between the first and second detecting electrodes 102 and 103 and the movable electrode 105 providing the capacitances C1 and C2 when an acceleration is applied is smaller than an amount of change d2 at the end of the movable electrode 105 having the weight 108. In other words, in view of a conversion efficiency of an acceleration due to an inertial force acting on the weight 108 to an amount of displacement of the movable electrode when the acceleration is applied, the conventional acceleration sensor cannot obtain a larger amount of displacement d1 of an interelectrode distance than an amount of displacement d2 of the weight 108. Therefore, a much larger amount of displacement of the weight is required than an amount of change in an interelectrode distance, which is required for obtaining a change in capacitance detectable by the detecting circuit side. This means that the rigidity of the deformation 107 is reduced more than necessary. A sensitivity to acceleration other than in a detecting axis direction may occur, which is not desirable as the sensor. The possibility that the movable electrode 105 is in contact with the semiconductor material 106 and/or the substrate 101 may be increased. Thus, the impact resistance and/or reliability of the sensor are disadvantageously reduced.
The weight 108 is required on the movable electrode 105 such that the movable electrode 105 can twist with respect to the deformation 107 when an acceleration is applied. However, the weight 108 is only provided at one end of the movable electrode 105. As a result, the center of gravity of the movable electrode 105 does not exist on the axis linking the deformation 107. Therefore, balance of the movable electrode 105 is difficult to obtain when no acceleration is applied. In other words, the movable electrode 105 twists even at the initial state. Therefore, a balanced state of the movable electrode 105 is hard to maintain. Thus, the same initial values of the capacitances C1 and C2 are difficult to obtain. As a result, the precision of detection may be reduced, and/or the step of calibrating a detecting characteristic may be complicated, disadvantageously.
Furthermore, the movable electrode 105 twists largely when an excessive acceleration is applied. Thus, the end may touch the substrate 101 and destroy the sensor structure.
In addition, no device is provided for correcting a characteristic changed due to a temperature change in an environment in use. Thus, an error may occur in acceleration obtained by the environment in use disadvantageously.
In view of the construction, the first detecting electrode 102, the second detecting electrode 103 and the driving electrode 104 are formed as embedded electrodes in the semiconductor material 106. The first detecting electrode 102, second detecting electrode 103 and driving electrode 104 and the metal contact 109 are connected electrically through the doped region 110. The depth of the first detecting electrode 102, second detecting electrode 103, driving electrode 104 as embedded electrodes and the doped region 110 in the semiconductor material 106 is limited by the processing technology physically. Due to the limitation and the detection principle, the flexibility in designing an amount of displacement of the movable electrode 105 decreases. Furthermore, the processing method is complicated, and the production cost increases disadvantageously.
The present invention was made in order to solve these problems. It is an object of the invention to provide a more reliable acceleration sensor for detecting an acceleration in a direction of a detection axis in a highly sensitive manner and for suppressing sensitivity to acceleration along other axes by improving the rigidity of the movable part.
It is another object of the invention to obtain an acceleration sensor having a construction with higher flexibility in design.
It is another object of the invention to obtain an acceleration sensor with higher impact resistance whereby the acceleration sensor is hard to damage when an excessive impact is applied thereto.
It is another object of the invention to obtain an acceleration sensor, which is small and inexpensive and can be mass-manufactured.
It is another object of the invention to obtain an acceleration sensor, which can detect accelerations in directions of three axes.